Discharge lamps, including high intensity discharge lamps such as high pressure sodium and the like, generally require high voltage pulses to initiate the ignition process within the lamp. High voltage pulses are applied to the lamp for starting and then, after the arc within the lamp is established, the supply voltage is lowered to an operating level, the actual level depending upon the characteristics of the particular lamp.
Among the circuits used for lamp starting are those which employ a tapped transformer, a discharge device, and a capacitor with a charging circuit, the capacitor being charged to a level at which it discharges through part of the transformer, creating the starting pulse or pulses.
Generally speaking, the magnitude of the starting pulse is established by selecting the parameters of the transformer and the characteristics and values of other components such as the discharge device and the capacitor. However, once these characteristics and values have been chosen, the circuit output in the starting mode is essentially uncontrolled.
It has been found that certain lamps, notably metal halide arc tubes, respond better to pulses having greater width. It appears that the scandium-iodide system with those lamps responds well to a pulse of lower magnitude but greater width. In addition, it is desirable to reduce the starting pulse to the lowest acceptable level in order to reduce the dielectric stress on both the lamp and ballast. Metal halide arc tubes are not placed in an evacuated environment in the way that other lamps, such as high pressure sodium. As a result, the possibility of breakdown within the lamp is much greater with metal halide. Reducing dielectric stress reduces the liklihood of lamp failure.
As a practical matter, alteration of the transformer ratio is an expensive matter and adds greatly to the cost of a system unless the transformer is going to be used for a large number of devices. Alteration of the values of the other circuit components has limited effect on the circuit output.
Furthermore, changing the turns ratio of the transformer is not a satisfactory solution. Changing the ratio to give the desired pulse width results in dropping the pulse magnitude. To compensate for this lower magnitude, the turns ratio would again need to be modified by adding more turns to the secondary which would decrease performance and also increase ballast size and cost.